The scientific Investigation of electroinag netic waves has resulted in a clear understand ing of the generation, propagation and detec tion of these waves and in the development of wireless telephony. The requirements of suc cessful radio-telephony began with a generator of undamped waves of very high frequency, so that the wave trains when received were above the range of audibility. Wireless telephony owes much to the work of the engineers of the American Telephone and Telegraph Company, who, in the latter part of 1915, succeeded in telephoning by wireless from Washington, D. C., to Paris, and from Washington to Hawaii, the latter a distance of 4,900 miles. The great ex pense of installations for long-distance work may limit competitive systems, but it is safe to look for the widespread employment of inde pendent short-distance wireless telephone sets.
Armies in the field employ portable wire less sets for insuring communication between scattered commands. Some sets for use in rug ged country are arranged to be carried on mule back and are lcnown as pacic sets; but the most common wireless sets are those mounted on two wagons, one for the generating equipment and the other for the wireless apparatus proper.
The European War gave especial impetus to wireless telephony, especially as applied to air craft. In designing radio apparatus for airships and aeroplanes due consideration must be given to the extremely limited space available on such craft and the limited weight that can be carried. On board airships of the Zeppelin or the flexible types it is possible to employ more powerful apparatus, hence a greater range can be covered.
A typical airship installation consists of a transformer, quenched spark gap, capacity and inductance, aerial wire lowered down from a winch, ammeter, rapid-change switch for differ ent wave-lengths and an alternating current generator driven off one of the engines of the airship. Such a set weighs about 55 pounds without the alternator and has a range between 60 and 120 miles. The aerial wire is over 600 feet long when fully paid out.